The visible light spectrum, known as photosynthetically active radiation (PAR), is of paramount importance in plant growth in that PAR light is responsible for driving photosynthesis. However, the role of ultraviolet radiation (UV) in promoting plant growth and survival is less well understood.
UV light itself is broken down into three spectral regions: the ultraviolet A light (UVA) is of wavelengths of between 320 and 400 nm; ultraviolet B light (UVB) is of wavelengths between 280 and 320 nm; and ultraviolet C light (UVC) is of wavelengths between 180 and 280 nm. The eat is bathed in both UVA and UVB light. However, UVC light is almost entirely filtered out by the earth's atmosphere.
Ultraviolet light in the UVB range is higher energy than UVA and responsible for damage to cells and tissues particularly with exposure to low wavelength UVB light. UVB radiation effects on plants that are attributed to susceptibility to pests, for example, include DNA damage (Britt, A. B., Trends Plant Sci, 1999; 4:20-25), modification in gene expression (Savenstrand, H. et al., Plant Cell Physiol, 2002; 43:402-10; Brosche M. and Strid A., Physiol Plant, 2003; 117:1-10), changes in secondary metabolism (Feucht W. et al., Vitis, 1996; 35:113-18; Picman A., et al., Verticillium albo-atrum Biochem Syst Ecol, 1995; 23:683-93; Glassgen W. et al., Planta, 1998; 204:490-98; Norton, R., J Agr Food Chem, 1999; 47:1230-35; Wicklow D., et al., Mycoscience, 1998; 39:167-72), and changes in leaf anatomy, i.e. leaf thickness and cuticle thickness (Garcia S., et al., Phytochemistry, 1997; 44:415-18; Liakoura, V., et al., Tree Physiol, 1999; 19:905-08; Raviv, M., and Antignus, Y., Photochem Photobiol, 2004; 79:219-26). Additionally, UVB light causes plants to produce UVB absorbing compounds, such as flavonoids and other phenolics, phenolpropenoids, alkaloids, and terpenoids. These secondary responses are generally independent of photosynthesis and produce photomorphogenic responses in UVB recipient plants.
Early experiments analyzing solar UVB effects demonstrated substantially reduced photosynthesis, plant growth, and crop yield. However, these early experiments were performed under unrealistic spectral balances in that high levels of UV were used. In experiments employing balanced levels of UV radiation, UV-induced partitioning of carbon to production of secondary plant metabolites occurs. Enhanced UVB radiation stimulates production of phenolics and flavonoids that serve a protective role by accumulating in leaf epidermal cells and attenuating UV radiation before encounters of sensitive processes in mesophilic cells. Synthesis of bulk methanol soluble UV absorbing compounds increases by 10% following enhanced UVB radiation.
Glasshouse manufacturers continue to claim that blocking UV radiation shows beneficial effects on reducing plant pathogens and insect pests. However, the presence of UV radiation, including high-energy UVB radiation, is actually beneficial to plant physiology and development. Indeed, when plants are subjected to UV light in addition to PAR many benefits are observed including insect and pathogen resistance and elevated levels of DNA repair capability. The reduced crop yields long thought to be the hallmark of increased ultraviolet light exposure, have recently been demonstrated to be inaccurate. The positive effects of UV radiation are not observed unless full spectrum light is present, including UVA, UVB, and photosynthetically active radiation. However, in radiation controlled studies in which UV and PAR are used simultaneously, high doses of UVB radiation relative to UVA causes some leaf damage in plants suggesting that the more unnatural the spectrum, the greater the damage caused by ultraviolet radiation. (Krizek, 1993; Caldwell, 1994.)
Gene expression is positively regulated by exposure to natural levels of ultraviolet radiation. As many as 70 UVB responsive plant genes have been identified that control mechanisms such as photosynthesis, pathogenesis, and the generation of antioxidants. Several processes regulated by UVB radiation are related to increased or enhanced plant color or fragrance. This modified genetic expression, translation, or modification pattern in the presence of UVB radiation partially explains why clones from the same plant grown in artificial lighting and sunlight look, taste and smell different than their genetic identicals grown in natural sunlight.
A majority of plants show significant benefit from ultraviolet light. Many of these are economically important plants such as herbs, drug producing plants, ornamental flowers, and food crops. Benefits of UV light include increased immune responses, enhanced pigmentation and aroma, and altered plant architecture such as shape, flower number and volume, and thricome density. A meta analysis of numerous plant species suggests that insect damage actually decreases with increasing doses of UVB light. (Bothwel, 1994; Mazza, 1999.) This response has been demonstrated in agricultural as well as in native plants. (Id.; Rousseaux, 1998.) For example, Isaguire, 2003 showed that expression of 20% of insect fighting genes of tobacco are increased after exposure to UVB radiation. These include proteinase inhibitors that inactivate insect digestive tract (Ryan, 1990) and furanocoumarin that results in slower development of insect larvae (McCloud, 1994). Production of insect repelling phenols is also observed following increased solar UVB radiation. (Fuglevand et al., 1996.) Defense to insects includes the formation of flavonoids or pigments that absorb UV in the 220 to 380 nanometer range. (Ormrod, 1995.) It is hypothesized that flavonoids and other chemicals produced in response to UV shield the plant by absorbing light in the UV range, inhibiting insect attachment and further scavenging free radicals.
Supplemental ultraviolet light on tomato plants produces a thickening of the skin that also increases resistance to insects such as boring insects. Other beneficial characteristics are simultaneously present such as the flavor of the pulp is considerably more complex and desirable. Fruit skin toughening is also found in naturally increased UV exposure. In analyses of plants in Tierra del Fuego on the southernmost tip of South America, which is regularly affected by severe ozone depletion increasing the levels of ultraviolet radiation exposure from the sun, insects prefer plant tissue before it is exposed to UVB light. (Ballaré, 2001.) An alternative hypothesis is that insects are refracted by the altered chemical production in leaves exposed to UVB light. In either case, UVB exposure is overall beneficial to plants.
Enhanced pigmentation is seen in many species after exposure to UV light. This is an important observation in that commercially important dyes are produced in plants. (Gilbert and Cooke, 2001.) The synthesis of dyes may be increased by exposure of these plants to UVB or UVA light. Also, enhanced pigmentation of ornamental flowers, especially noted in blue, black and purple tones, is seen after exposure of these plants to UVB light. (Kevan, 2001.) The increase in pigmentation as well as flavonoid production are well documented in response to ultraviolet light. Increase in flavonoid compounds is attributed to the beneficial effects in fruit, vegetables, tea, and red wine grapes to name a few. A specific non-limiting example is the production of anthocyanin that is increased in response to ultraviolet light exposure. The presence of anthocyanin causes roses to appear red to blue depending on the pH in which they are grown. This is seen most often in the leaves of juvenile plants as a reddish hue which disappears as the new leaves mature. However, increased anthocyanin production requires high levels of photosynthetically active radiation alongside increased UV levels. (Steyn, 2002.) As such, a gardener growing roses will see this effect when both UV and photosynthetically active light are present in optimal conditions.
Aromatic oils are also enhanced by in output by ultraviolet light. Such oil output is increased in basil and mint, for example. (Johnson, 1999.) These essential oils are concentrated in glandular thricomes which appear to benefit significantly from UVB radiation.
Auxin levels which absorb UVB light are photo degraded by levels of UVB. However, ethylene which causes radial growth and less elongation in plants is increased after UVB irradiation in sunflower seedlings (Ross and Tevini, 1995) and pear seedlings (Predieri et al., 1993).
UV light exposure increases expression of many of the greater than 25,000 terpenoids known with many with diverse functions in plants. Anti-insecticidal activity is achieved by increased levels of pyrethin which is a natural insecticide (Harbourne, 1991). Beneficial insects may also be attracted by terpenoids such as pollinators and predatory wasps.
Alkaloids are found in 20% of flowering plants are enhanced by UV light. Greater than 12,000 different alkaloids are known in plants and may be economically important as pharmaceuticals including morphine, nicotine, caffeine and cocaine. They are also important as insecticides and other deterrents. Indeed nicotine from tobacco was one of the first insecticides deployed by humans. Phenolic compounds, which illustratively include coumarins, furanocoumarins, and flavonoids, are also produced in the presence of ultraviolet light. There are more than 4,500 flavonoids known. Many flavonoids are strongly colored and used by plants and flowers and fruits to promote pollination and seed dispersal. Thus, UV induced phenolic compounds increases the ornamental desirability of flowering plants or fruits.
Finally, plant shape, architecture, flower number, and thricomes are enhanced or affected by the presence of ultraviolet B radiation. For instance, both UVA and UVB inhibit stem elongation in a wide variety of plants. Decreased elongation is attributed to UV induced destruction of the plant hormone auxin, however increases in the hormone ethylene cause greater radial growth and less elongation, as has been seen in sunflower seedlings and pear seedlings. (Ros and Tevinin, 1995; Prediere, 1993.) These architectural effects may be exploited to improve handling and growing procedures for crops. For example, lower levels of stem elongation can allow for greater stacking capability within a single greenhouse or light irradiated structure increasing crop yield per unit area. Also, balanced light conditions consisting of PAR in addition to UV radiation produce an increase in flower numbers inside a glasshouse. (Grammatikopoulos, 1998; Day, 1999.) Increased diameter of the flowers is also achieved. (Petropoulou, 2001.) In general monocots are more responsive to increased levels of UVB in glasshouses illuminated as in the instant invention than are dicots. (Barnes, P., Am J Bot, 1990; 77:1354-60.)
These studies suggest that plants are improved in their yield or ornamental appearance by a proper balance between UV radiation and other light wavelengths. The problem is that high intensity bulbs are limited in the amount of ultraviolet radiation produced to approximately 3% of the total light output. (ANSI C78.38-2005) Therefore, given that UV light is both beneficial to photomorphogenic properties of plants and harmful if used improperly, there is a need for a process of irradiating plants using an artificial light source so as to improve the growth, appearance, disease resistance and desirability of the plants and their fruit.